Surly are our friends: they make excellent cheap functional products for singlespeed and fixed gear, they answer emails, and they let us reprint their articles. This example originally contained references to their very popular Singleator product. We removed them for safety reasons. Chain tensioners are for singlespeeders. You cannot use them on fixies. We didn't use <snip>s to show what we cut, but please note that the article is a little different from Wake's original.
One-speed offroad drivetrain spew
by Wakeman Massie, Surly
A lot of you are new to singlespeed/fixed gear and haven't identified the idiosyncrasies of the one-speed drivetrain and how to build and maintain a good one. No worries, we love you, but just because one-speeds are simple and pure and beautiful and wonderful and all that doesn't mean you get to slap one together, neglect it and expect everything to work correctly. In the interest of furthering public knowledge and reducing the number of confused-customer-calls we receive, here's some general information about one-speed drivetrains.
The Micro-drive concept is bullshit on a single-speed. 'Micro-drive', a term originally coined and trademarked by the dear departed Maeda Corporation (Suntour), is a system in which smaller chainrings and cogs are used to achieve a given gear ratio than would previously have been possible. Micro-drive systems are based on 11t first position cogs, which only became practical with the development of high-quality heat-treated steels. Matching lighter, smaller cogs and chainrings to shorter derailleur cages and chains gave the weight freaks something to jump up and down about, and the system actually sorta worked for the rest of us. Shimano copied and the rest is history -- say hello to modern multi-speed MTB transmissions.
Micro-Drive works because, on a multi-speed bike, you can get away with smaller cogs for higher gears. Most of us never spent much time in those 11- and 12-tooth cogs, so they didn't wear all that fast. And when we did use them, it wasn't full-bore high-throttle. If it ever got so hard to pedal that you needed to stand up, you simply shifted to an easier gear.
On a one-speed, it's a different matter. If you were to take a 22x11t onespeed offroad, you'd be launching the chain every time you got up and pushed, no matter how new your components were. In case you didn't realize, only about half the teeth on your cog engage the chain at any given time. On an 11 tooth cog, that's only 5 or 6 teeth! Unless your thighs are as big around as a Surly carpenter pencil, you will be able to make any low-gear transmission based on an 11t cog skip AT WILL.
In fact, it's not so different for a 16t cog--about the smallest you should consider for a one-speed ride. You'll be giving EIGHT teeth the full wrath of your wide load up the harshest vertical climbs you choose to attack. And, since you never leave that gear, every mile you spin will be on those same few teeth.
Your drivetrain will last longer, skip less, and launch the chain fewer times if you use larger cogs and chainrings. The weight penalty is practically non-existent; there is no benefit whatsoever to using a Micro-Drive drivetrain on your one-speed off-road bike. Don't do it! It's dumb and you're gonna hurt yourself on a steep climb.
We recommend that you pick a big cog out back (18 teeth or larger) and experiment to find a gear you like by varying your chainring size up front.
Chainline refers to the alignment of the front chainring and the rear cog. Chainline is important on all bicycle drivetrains but especially so on a one-speed. A perfect chainline is one in which the chainring and cog are so closely aligned that the chain takes a straight path between them. Anything less than damn close on a one-speed will result in premature wear and frequent chain launching. Chain line can be fine-tuned by re-spacing your hub, adding or deleting freewheel spacers or cassette spacers, changing cranks, changing BB spindle lengths, shimming bottom brackets, using spacers between the crank spider and chainrings, and anything else you can think of to get your ring and cog to line up nicely. Your drivetrain will be quieter, too.
If your chain is too tight, it will wear quickly and be harder to pedal. If it is too loose, it will throw itself off on the downhills and skip over the top of the cog and/or chainring on the steep uphills. You may have noticed when trying to get your chain tension dialed in that the chain is tight in some spots and loose in others. This is normal(sic). Chainrings and crankarms bend, warp and deform in use, and generally aren't perfectly round to begin with. I like to adjust my chain so that the chain still runs freely when it's at its tightest spot. That's kind of hard to explain clearly but suffice to say you won't have any catastrophic problems if you can keep the tension somewhere between 'so loose the chain is flopping around on the downhills' and 'so tight you can feel resistance when you move the pedals with the bike upside down.'
For those of you with bolt-on hubs, greasing the axle nuts or bolts will help keep unwanted wheel movement to a minimum while you're trying to get your tension set just right. Tightening left and right bolts a little at a time helps, too. Good luck.
1/8" chains vs. 3/32" chains
1/8" chains suck. Run whatever you want, but bigger isn't better here. Yeah, they're wider, but according to manufacturer-supplied data, they're not stronger and they're definitely not of better quality. Multi-speed drivetrains is where the bucks are at, and chains that work on such drivetrains are where the manufacturers showcase their innovations and developments in quality. The rollers are better, the plates are better, the pins are better, and the construction method is better on all multi-speed 3/32" chains. I guess if you grind your chainring and chain down the handrail every night at the local pub, a bigger 1/2x1/8 " chain will last longer, but most of us don't and it won't.
Cogs! (or, sprockets!)
It is possible to buy cheap cogs laser-cut from thin metal. They're flexible, meaning that they throw your chain when you least want it. And they have weak threads that bust out under pressure, often stripping the thread from your hub at the same time. Your cog is pretty important: it just doesn't make sense to skimp. We make Sub-11.0 cogs out of stainless steel.
Hey, REPLACE PARTS OFTEN!!
You will have to. Let's say the average dude spends most of their time in 5 or 6 different gears (out of 27, sheesh!) on any given multi-speed bike ride. You spend all your time in one gear. And you're wrenching the snot out of those drivetrain parts in a way they weren't designed for. Your parts will last 1/5 or 1/6th as long as the average dude's. Get used to replacing transmission parts often!
Hey, USE NEW PARTS TOGETHER!!!
Just like on multi-speed drivetrains, used cogs and rings don't like new chains and vice-versa. If you mix 'n' match new and old drivetrain parts, particularly cogs and chains, I hope it's on your urban luxo-cruiser. When you take your one-speed ride through technical stuff and up big, steep hills that almost stop you in your tracks, you're pushing your drivetrain to the absolute limit.
Don't believe me? Ask a friend of similar weight and strength to you to ride their multi-speed 25 times hard with a new chain, and then measure the amount of stretch from the center of one pin to the center of another pin 15-or-so links away. (Remember, every link is supposed to be exactly 1/2" long from pin center-to-pin center.) Do the same thing with your single-speed-- new chain, 25 hard offroad rides. You won't believe how much more your chain stretches!
By replacing cog, chain and ring at the same time, you ensure that the teeth and chain wear together to ensure a good, strong mesh that won't skip under load. If you mix new and old, you won't get the symbiotic (ulp?!) wear patterns that allow tooth and chain to mesh together properly. You will get funky 'ticks' and 'pops' and skipping under load. This is age-old wisdom here.
Hey, GEAR LOWER!!!!
I know all you guys and gals out there are gorillas and want desperately to outdo your pals mentally, socially, physically, etc, but from a functional standpoint lower gears are better on a one-speed off-road drivetrain. I suppose if you're a member of the go-fast club you have to run a big gear, so you HRM-types can skip this part. But if you're more concerned with your ability to ride and clean the tight stuff, you'll be doing your style a favor and you'll be sparing your drivetrain a measure of unnecessary abuse by sticking with low(er) gears. Yes, you will have to spin more on the flats, but if you gear to avoid herniating yourself up the climbs you will also reduce the effects of wear and shock and the potential for catastrophic breakage on your drivetrain. Not to mention your unmentionables.
Our one-speed mountain bike or cross bike has probably been designed to be at least reasonably light... even Surly folk don't enjoy riding 30lb. bikes up hills. (On the other hand, a 3.5 lb. cro-moly frame is not the best choice for a one-speed, and if you're in this boat, I hope you weigh under 150lbs.) Most likely you have a light frame with thin chain- and seat-stays that are anywhere from 16-20" long. Every time you push 'n' pull on the pedals, the bottom bracket swings back and forth and the long and skinny stays get pulled toward the drive side of the bike. The whole machine is gettin' twisted by your upper and lower body, especially so on the tough climbs. To a certain degree this flex is desirable and produces a comfortable ride, but in the drivetrain it creates deviations that can cause skippage, premature wear and chain launching. This is not something to be ignored!
The best way to deal with flex-related problems is to make sure everything else is in order. Make sure your chainline is perfect, your rear cog is big, you're geared low enough, your drivetrain components are matched, and your chain is tensioned properly. That way you minimize the potential for frame flex to disrupt things.
Lube your chain.